Devices of this type that have heretofore been known include such devices as disclosed in Japanese Patent Publications Nos. 22208/78 and 32889/79. The former device uses a retainer of an integral structure having ball turning grooves formed in end portions thereof. Assembly and disassembly are difficult because the balls must be moved into and out of the spline in the radial direction. Furthermore, the shape of an outer race is complex. The latter device is also disadvantageous in that its outer race is required to have a large wall thickness because the ball circulation paths extend radially with respect to the spline.